1. Field of the Invention
This invention relates to a power line communication that performs the communication by using a power line.
2. Description of the Related Art
ECHONET, Japanese standard of the network using in-house power lines, has been a focus of constant attention. ECHONET is the acronym for Energy Conservation and Homecare Network, contents of which are explained hereunder according to the written standard of ECHONET, Ver.1.01, (published by ECHONET Consortium).
The architecture of ECHONET is shown in FIG. 12.
In ECHONET, electric devices in one range in managing properties, security, and the like, are generally put together into a system. The territory covered by the largest unit in ECHONET is referred to as a “domain”. In other words, the domain is defined as the territory of controlled resources (home equipment, appliances and consumer electronics, sensors, controllers, remote controls, etc.) in the range of the network defined in ECHONET.
In ECHONET, an electric device is in communication with another device or a controller for monitoring, controlling, and operating electric devices. And, the electric devices and the controller work in concert with each other. That is called a “system”. The range of a system exists in one domain and does not extend over a plurality of domains. One and more systems may be included in the same domain. When a system is connected to another system outside the domain, the system is required to be provided with an ECHONET gateway. Therethrough, the system can be connected with the external of the domain.
ECHONET is allowed to use plural types of transmission media and protocols so as to enable the best use of the characteristics of various transmission media and the construction of optimal systems. The ECHONET network configuration model for the main transmission media is shown in FIG. 13.
As shown in FIG. 13, the connection outside the domain is established through ECHONET gateways (GW). And ECHONET routers, which are intervened between different protocols (i.e., different, transmission media) within the same domain, establish the connection between them.
Respective networks divided by the ECHONET routers are defined as a “subnet”. Inserting an ECHONET router to a subnet with the same protocol enables to create a different subnet. The network configuration of a domain can be represented as a collection of subnets. In other words, the domain is a part of the configured network including ECHONET routers, wherein in-house information is exchanged.
In a subnet, a node's identifier (Node ID) is defined and used as an ECHONET communication function identifier unique within at least the subnet (defined as an ECHONET Node). Each subnet has its own unique subnet identifier (Net ID). An ECHONET address is represented as a pair of the subnet identifier and the Node identifier, and this pair is used as the ECHONET Node identifier unique within the domain.
Next, the ordinary power line communication waveform is shown in FIG. 14.
An ordinary power line communication through in-house power lines, like the ECHONET-compliant communication, superimposes a carrier signal waveform 2002 on a commercial power waveform 2001, by which data is transmitted. The frequency of the commercial power wave is 50 Hz or 60 Hz, and the frequency of the carrier signal wave is between 10 kHz and 500 kHz.
Since the above-mentioned power line communication may use an ordinary existing power line, no new cable is required to be wired between in-house electric devices. However, the power line communication has following problems.
First of all, an ordinary in-house power line is not designed for the data communication, and connected to a number of electric devices irrelevant to the power line communication. Additionally, the length of the in-house power line becomes a few 10 meters or a few hundred meters. Those cause the significant deterioration of signals. Due to such significant signal deterioration, the transmission rate of signal becomes 600 bps at most, which is very low.
Secondary, since the electric devices with low impedance are connected to the power line, the signals are absorbed by those electric devices. Therefore, the electric device performing the power line communication needs a big transmission power, e.g., 100 mW.
In addition, when a number of electric devices compliant with the power line communication (called a “power line communication-compliant device) are connected to the power line, all the signals flow into the in-house power line. This causes a problem of a shortage of the communication capability.
Moreover, even if there is the information that the user does not want to leak from a specific device by any means, since all the signals flow into the in-house power line, it is possible for the other in-house devices to obtain the information.
In order to settle the above-mentioned problems, for instance, Japanese Patent Publication No. 7-231284 discloses the following power line carrier system.
Specifically, as shown in FIG. 15, a power strip 2101 receives the power supplied from an external power line 2110, and connects electric devices 2107a, 2107b, and 2107c, those devices to be a communication object. The power strip 2101 is provided therein with a block filter 2105, thereby the signal among the electric devices 2107a, 2107b, and 2107c is blocked entirely, against the external power line. Accordingly, the above-mentioned problems caused from the in-house external power line can be settled regarding the communication among the electric devices 2107a, 2107b, and 2107c. 
However, the above conventional power line carrier system has following problems.
First, a communication object in the conventional power line carrier system is limited to an electric device connected to the same power strip. That is to say, if either of electric devices is not connected with the same power strip, the electronic devices do not perform any communication mutually. To enable the communication between the devices that are not connected to the same power strip, it is necessary to wire in a house a new power line corresponding to the high-speed communication, which causes the increase of the user's burden.
Additionally, in the conventional power line carrier system, the signal is sent to all the electric devices connected with the same power strip. That it to say, since the signal is sent to even non-communication-object electric devices, there is a problem of the security of the information as well as there is a possibility that the communication processing delays.
Moreover, it is necessary for the, conventional power line carrier system to prepare a power strip with a block filter other than, the communication object electric device. In other word, this becomes a problem that the facility of the power line communication, such as only to plug into the power outlet, is thrown away.